Apparatus for generating a signal train dependent on rotary speed

ABSTRACT

An apparatus is proposed for generating a signal train dependent on rotary speed from the periodic voltage of an inductive transducer arrangement, especially for controlling ignition systems in internal combustion engines. The transducer voltage is delivered to a threshold stage. In order to reduce an inherent systematic error, means are provided which shift the transducer voltage, before it is supplied to the threshold stage, in accordance with amplitude in such a manner that the switching point of the threshold stage is invariant for a given rotary angle.

The present invention relates generally to inductive transducers used inmotor vehicle ignition systems, and more particularly to an improvedcircuit for deriving a digital output from the signal produced by thetransducer.

It is known, especially in ignition systems, to use inductive transducerarrangements, which supply voltages dependent on rotary speed. Thesevoltages are delivered to a threshold stage, which furnishes a signaltrain dependent on rotary speed. As the rotary speed increases, theamplitude of the voltage emitted by the transducer normally increases aswell. At a constant threshold value of the threshold stage, the range ofshaft angle at which the threshold value is exceeded varies as aconsequence of the increase in amplitude. The result is that theignition system in which the transducer arrangement is used exhibits aninherent systematic error.

THE INVENTION

The apparatus according to the invention and having means for shiftingthe transducer voltage signal, in accordance with amplitude, before thevoltage signal is applied to the input of the threshold stage has theadvantage over the prior art that, as a result of the shift in voltageof the transducer signal, the thus-modified input signal of thethreshold stage crosses over or under the threshold value at the samerotary angle despite varying amplitudes of the transducer voltages. Aninherent systematic error on the part of the ignition system is therebyavoided or sharply reduced.

DRAWINGS

FIG. 1, a circuitry realization of the exemplary embodiment;

FIG. 2, voltage courses for the apparatus according to the invention,plotted in accordance with time; and

FIG. 3, transducer voltages and the output voltage of the thresholdstage, plotted in accordance with the rotary angle.

DETAILED DESCRIPTION

An inductive transducer arrangement 1, which can preferably be connectedwith the crankshaft or camshaft of an internal combustion engine or withsome other shaft of an engine, is connected via a low-pass filter 2 withthe cathode of a first diode 3 and the anode of a second diode 4, afirst resistor 5 and a second resistor 6. Taken in order from the anodeof the first diode 3, a first capacitor 7 is connected to ground, and afirst discharge resistor 8 is located parallel to it. A third resistor 9which is connected with the first resistor 5 is also connected with theanode of the first diode 3. The junction A of these two resistors 5, 9is connected via a third diode 10 with the input of a threshold stage 11embodied as a Schmitt trigger; the anode of the third diode is locatedat the junction A and the cathode is located at the input of the Schmitttrigger 11. Taken in order from the cathode of the second diode 4, asecond capacitor 12 is connected to ground, and a second dischargeresistor 13 is located parallel to it. The cathode of the second diode 4is furthermore connected via a fourth resistor 14 with the secondresistor 6. The junction B of these two resistors 6, 14 is connected viaa fourth diode 15 to the input of the Schmitt trigger 11, and thecathode of the fourth diode 15 is located at the junction and the anodeis located at the input of the Schmitt trigger 11. The junctions betweenthe first and third resistors 5, 9 and between the second and fourthresistors 6, 14 are connected to ground via protective diodes 16-19. Theresistors 9 and 14 are approximately twice as large as the resistors 5and 6.

As shown in FIGS. 2 and 3, by means of the inductive transducerarrangement 1, an alternating voltage is generated in accordance withthe rotary speed; this voltage is shown in FIG. 2 plotted in accordancewith time and in FIG. 3 in accordance with the rotary angle. Thetransducer voltage is carried via the low-pass filter 2, in order toeliminate the high-frequency peaks. The first capacitor 7 charges viathe first diode 3 to approximately the negative peak value of thetransducer voltage and then discharges, with the first dischargeresistor 8 and the third resistor 9 determining the discharge speed. Thedischarging process is represented in FIG. 2 by the curve 21. Thetransducer voltage is carried further via the first resistor 5 to thejunction A, at which the transducer voltage 20, at twice the value, andthe discharge voltage 21 are superimposed. This is shown in FIG. 2 bythe curve 22, which represents a shift of the transducer voltage tonegative values. The third diode 10 is connected in such a manner thatonly positive voltages are allowed to pass through. This range isindicated in FIG. 2 by shading. The threshold value of the Schmitttrigger 11 is preset in such a manner that the Schmitt trigger switchesat the instant when the curve 22 crosses from negative voltages throughzero.

The same applies to the negative half of the wave of the transducervoltage 20. Via the second diode 4, the second capacitor 12 is chargedpositively up to approximately the peak value of the transducer voltage.At the junction B, the voltages are superimposed on one another, and theresult is the curve marked 23 in FIG. 2. The fourth diode 15 allows onlynegative voltages to pass through, and the Schmitt trigger 11 switcheswhenever the voltage 23 crosses through zero from positive values. Viathe protective diodes 16-19, a portion of the current is drawn offoutside the switching range of the Schmitt trigger 11.

Normally, the amplitude of the transducer voltage increases inaccordance with the rotary speed. This higher transducer voltage isindicated by dashed lines in upper part of FIG. 3. If the amplitude ofthe transducer voltage 20 is increased, the capacitors 7, 12 are alsocharged up to higher peak values; that is, the discharge voltages 21, 24have higher absolute values. In the superimposition of the voltages atthe junctions A, B, it is demonstrated that as a result of the shift ofthe discharge voltages 21, 24 to higher absolute values, the zerocrossing of the voltages corresponding to the curves 22, 23 remainsconstant in accordance with the rotary angle. This is shown once againin FIG. 3. The lower part of FIG. 3 shows the output voltage of theSchmitt trigger 11 in accordance with the rotary angle. It can be seenthat the switching point 25 on the dashed line shifts linearly up to theswitching point 26 in accordance with the higher transducer voltage.

In the present exemplary embodiment, the negative discharge voltage 21is utilized for shifting the positive wave half of the transducervoltage 20, thereupon already attaining sufficient precision. For stillmore precise systems, the positive discharge voltage can be utilized forshifting the switching point of the positive wave half of the transducervoltage 20 as well, for instance if an inverter is connected between thejunction of the resistors 5 and 6 with the low-pass filter 2 and thejunction of the diodes 3 and 4.

I claim:
 1. An apparatus for generating a digital signal trainrepresentative of rotary speed from the periodic voltage of an inductivetransducer arrangement, in particular for controlling ignition systemsof an internal combustion engine,said signal generating apparatus havingan inductive transducer arrangement (1), a voltage signal superpositionstage (3-10, 12-19) connected to said transducer (1), and a thresholdstage (11) connected to said superposition stage, wherein, in accordancewith the invention and in order to reduce the inherent systematic errorwhich can result from variation of the amplitude of transducer outputvoltage as a function of rotary speed, means are provided which shiftthe transducer voltage to a different absolute value in accordance withsaid output voltage amplitude, before it is delivered to the thresholdstage (11), in such a manner that the switching point of the thresholdstage (11) is invariant for a given rotary angle, regardless of theamplitude of said voltage.
 2. An apparatus as defined by claim 1,characterized in that via a first diode (3, 4) a capacitor (7, 12) ischarged by the transducer voltage, that the voltage located at thecapacitor (7, 12) is superimposed with the transducer voltage and thatprior to the threshold stage (11) a further diode (10, 15) is provided,which rectifies the superimposed voltage.
 3. An apparatus as defined byclaim 2, characterized in that the first diode (3, 4) and the furtherdiode (10, 15) have opposite conducting directions.
 4. An apparatus asdefined by claim 1, characterized in thata first separate circuit path(4,12,15) is provided for the negative half-oscillation of thetransducer voltage and a second separate circuit path (3,7,10) isprovided for the positive half-oscillation of the transducer voltage,each circuit path having a first diode (3,4), one capacitor (7, 12) andone further diode (10,15).
 5. An apparatus as defined in claim 4,further comprising a resistor (5,6) connected in parallel with the firstdiode (3,4) of each of said circuit paths.
 6. An apparatus as defined byclaim 2, characterized in thata first separate circuit path (4,12,15) isprovided for the negative half-oscillation of the transducer voltage anda second separate circuit path (3,7,10) is provided for the positivehalf-oscillation of the transducer voltage, each circuit path having afirst diode (3,4), one capacitor (7, 12) and one further diode (10,15).7. An apparatus as defined in claim 6, further comprising a resistor(5,6) connected in parallel with the first diode (3,4) of each of saidcircuit paths.
 8. An apparatus as defined in claim 3, characterized inthata first separate circuit path (4,12,15) is provided for the negativehalf-oscillation of the transducer voltage and a second separate circuitpath (3,7,10) is provided for the positive half-oscillation of thetransducer voltage, each circuit path having a first diode (3,4), onecapacitor (7, 12) and one further diode (10,15).
 9. An apparatus asdefined in claim 8, further comprising a resistor (5,6) connected inparallel with the first diode (3,4) of each of said circuit paths. 10.An apparatus as defined in claim 1, wherein said threshold stage (11) isa Schmitt trigger and is preset to switch when the output voltage ofsaid superposition stage crosses though zero.
 11. An apparatus asdefined in claim 10, furthercomprisingprotective means (16-19) fordrawing off current outside the switching range of said Schmitt trigger(11).
 12. An apparatus as defined in claim 2, wherein two of saidfurther diodes (10,15) are provided, and the anode of one (4) of saiddiodes and the cathode of the other (3) of said diodes are bothconnected to an input of said threshold stage (11).
 13. An apparatus asdefined in claim 1, further comprising a low-pass filter (2) connectedbetween said transducer (1) and said superposition stage (3-10,12-19).14. An apparatus as defined in claim 2, further comprising a low-passfilter (2) connected between said transducer (1) and said superpositionstage (3-10,12-19).
 15. An apparatus as defined in claim 3, furthercomprising a low-pass filter (2) connected between said transducer (1)and said superposition stage (3-10,12-19).
 16. An apparatus as definedin claim 4, further comprising a low-pass filter (2) connected betweensaid transducer (1) and said superposition stage (3-10,12-19).
 17. Anapparatus for generating a digital signal train representative of rotaryspeed from the periodic output voltage of an inductive transducerarrangement, in particular for controlling ignition systems of aninternal combustion engine,said signal generating apparatus having aninductive transducer arrangement (1), a low-pass filter (2) connected tosaid transducer (1), a voltage signal superposition stage (3-10, 12-19)connected to said low-pass filter (2), and a Schmitt trigger thresholdstage (11) connected to said superposition stage, wherein, in accordancewith the invention and in order to reduce the inherent systematic errorwhich can result from variation of the amplitude of transducer outputvoltage as a function of rotary speed, means (3-10,12-19) are providedwhich shift the transducer voltage to a different absolute value inaccordance with said output voltage amplitude, before it is delivered tothe threshold stage (11), in such a manner that the switching point ofthe threshold stage (11) is invariant for a given rotary angle,regardless of the amplitude of said voltage, said shifting meanscomprising a first separate circuit path (4,12,15) for processing thenegative half-oscillation of the transducer voltage, and a secondseparate circuit path (3,7,10) for processing the positivehalf-oscillation of the transducer voltage, each circuit path having afirst diode (3,4), one capacitor (7, 12) and one further diode (10,15)connected to the input of said Schmitt trigger stage (11).
 18. Anapparatus as defined in claim 17, further comprising a resistor (5,6)connected in parallel with the first diode (3,4) of each of said circuitpaths.